Capture Mechanism and Capture Process

Brian  Wilcox,  Capture  Mechanism  Lead  

Key Characteristics of Asteroid for Capture

Composi8on/Strength  

Rock  (>>1PSI)  

Dirt  Clod  (~1PSI)  

Rubble  Pile(

Spin Periods of Near-Earth Asteroids

•  Many small NEAs spin faster than the rubble pile spin barrier, but may be "dirt clods“

•  Worst case assumed to be 5-13 m diameter NEA with a spin rate of 2 RPM and tumbling

3

Easy  

Harder  1  RPM  

Envelope  of  candidate  ARRM  targets  

Capture Mechanism Concept

• Capture bag formed of cylindrical barrel section and conical section attached to S/C

•  Inflatable exoskeleton to deploy bag after arrival at asteroid

• Circumferential cinch winches close diaphragm at top of cylindrical section and confine asteroid after deflation

• For fast-spinner, air bags quickly immobilize bag in asteroid frame at very low contact pressure (

Slow Spinner Capture Sequence

•  S/C approaches, no need to match rotation state (upper bound of spin rate for this mode has not been determined)

•  When asteroid is centered in the bag, close top diaphragm, begin controlled winching process, cinch asteroid tight to S/C while venting.

Fly  S/C  to  posi8on

 bag  over  asteroi

d,  close  diaphragm

 over  top,  winch  b

ag  down  

5

Fast Spinner Capture Sequence

•  S/C approaches and matches spin along projected asteroid spin vector a short time in the future.

•  When asteroid is centered in the bag, close top diaphragm, and at the moment spin is matched, inflate air bags w/pressure

• Asteroid inertial and spin properties determined by observation and state accurately projected into the future by many minutes to hours

• Asteroid instantaneous spin vector circulates around angular momentum vector

• Spinning S/C approaches along projected instantaneous spin vector and grabs when vector matches S/C location to minimize bag scuffing

Passive Capture, Matched Instantaneous Spin Vector

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Monte Carlo Simulation Results

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Requirement:  ≤1,765  Nm  

x  

z  y  

x  y  

SADA  

z  

•  5,472  samples  (4  spin  rates,  36  shapes,  and  38  fully  nutated  states)  •  Grab  uses  op8mized  generalized  6-‐DOF  spring-‐loaded  joint  •  Spring  parameters  are  derived  from  an  op8miza8on  within  the  base-‐line  toroidal  isola8on  device  design  constraints  

 

SADA  =  Solar  Array  Deployment  Actuator  (the  limi8ng  component  for  S/C  accelera8on)  

Capture Testbed

• Ques8ons  that  will  be  answered  by  scale  model  and  full  scale  tes8ng  (not  likely  answerable  by  physics-‐based  simula8on)  include:  

–  Cinch cords behavior and control of bag fabric, demonstrating full closure of the bag?

–  Characterizing snagging of the bag by the asteroid, forces on the bag, and general control of the bag

–  Determining the best cinching and GN&C algorithms to manage the asteroid motion in the bag 9

•  Ini8al  1/5  scale  capture  testbed  has  inflatable  exoskeleton  with  winches  suspended  from  gantry  over  asteroid  on  end  of  8-‐DOF  robot  arm  that  can  spin  and  tumble  the  asteroid  in  the  S/C  (lab)  reference  frame.  

•  Force-‐torque  sensor  at  aeachment  of  asteroid  to  arm  allows  realis8c  spin  physics.